In the field of oil and gas production, the design of new wells has become a critical factor in minimizing drilling cost and in ensuring successful production. Proper well planning includes such factors as the weight of the drilling mud to be used during the drilling operation, and the necessity for and design of casing strings. The selection and planning of mud weight and casing depend primarily upon the pore pressure of fluid bearing formations below the surface of the earth.
On one hand, the drilling mud weight cannot exceed the pressure at which a formation will fracture. Fracture of the formation at the wellbore can cause loss of circulation of the drilling mud, since the mud can escape from the wellbore into the surrounding lithology. Accordingly, low pressure zones that are vulnerable to such fracturing must be detected, and the appropriate precautions taken, so that such fracturing and loss of mud circulation does not occur.
Conversely, if the mud weight is significantly less than that of the pore pressure of a formation, a disastrous "blowout" condition can occur, in which the fluid from the formation pushes the drilling mud out of the wellbore. As such, a particular problem in the design and execution of a well is the possibility of encountering so-called "overpressurized zones" in the subsurface geology.
As is well known in the art, hydrostatic pressure increases with depth from the surface. Since fluid is not compressible, any fluid located in permeable rock formations tends to become squeezed out therefrom as the hydrostatic pressure increases with increasing depths. However, if surrounding formations "seal" a region of permeable rock, the fluid contained therein cannot be expelled from the region. As more and more sedimentation accrues over geologic time, and thus as the overburden increases, the fluid in these regions carry more and more "weight"; in many cases, the trapped fluid supports the formation, which is indicated by greater than typical porosity for the formation. Since the resulting fluid pressure in these zones are abnormally large as compared to zones where the fluid has been above to migrate, these zones are referred to in the art as "overpressurized zones". These zones are also referred to as "geopressure" zones, since the pore pressure in these formations resulting from the trapping of fluids is greater than the hydrostatic pressure at the corresponding depths.
Proper well planning becomes even more complicated where overpressurized zones underlie low pressure zones, as is typical in certain parts of the world; in this condition, if the drilling mud weight is increased to the extent necessary to avoid a blowout, this mud weight can fracture the overlying low pressure zone. In this case, the well plan will require casing strings to be set at the intermediate depths of the low pressure zones to protect those formations vulnerable to fracturing from the pressure of the heavier drilling mud. Factors such as the depth of the settings for the casing, the casing weight, size of the hole, and the like must also be selected. Of course, while avoidance of blowouts is of primary concern, the use of intermediate casing strings in a well is quite costly, and is therefore preferably minimized to the extent necessary.
Accordingly, as is well known in the art, the estimation of pore pressure at varying depths below the location of a proposed well is essential in proper well planning. Several methods are known in the art for estimating pore pressures in formations, using well log data and also from seismic survey information.
One such method is well known in the art as the "Eaton" method, and is described at Eaton, "The Equation for Geopressure Prediction from Well Logs" SPE 5544 (Society of Petroleum Engineers of AIME, 1975). The Eaton method of determining pore pressures begins with determination of the so-called "normal compaction trend line" based upon sonic, resistivity, conductivity, or d-exponent data obtained by way of well logs. The normal compaction trend line corresponds to the increase in formation density (indicated by sonic travel time, resistivity or conductivity) that would be expected as a function of increasing depth due to the increasing hydrostatic pressure that forces fluids out from the formations and thus increases sonic travel time, assuming the absence of geopressure. This normal compaction trend line may be determined solely from the sonic travel time, conductivity, or resistivity well log data, or may be adjusted to reflect extrinsic knowledge about the particular formations of interest. The Eaton method calculates pore pressure by correlating the measured density information, expressed as an overburden gradient over depth, to deviations in measured sonic travel time, (or electrical resistivity or conductivity) from the normal compaction trend line at specific depths. The pore pressure calculated from the Eaton equations has been determined to be quite accurate, and is widely used in conventional well planning.
However, application of the Eaton method has been limited to the immediate locations of existing wells, as it depends on well log data. It is of course desirable to estimate pore pressure at locations at the sites of proposed new wells, and thus away from currently existing wells, particularly to identify locations at which production will be acceptable at a low drilling cost (e.g., minimal use of intermediate casing). In addition, knowledge of pore pressure at locations away from existing wells enables intelligent deviated or offset drilling, for example to avoid overpressurized zones.
Another well known method of estimating pore pressure during drilling correlates pressure gradient information determined from the mud weight profile in the well with deviations in seismic two-way transit times from that expected by the normal compaction trend line. Regression techniques are then used to derive a predicted pore pressure to interval transit time relationship. As described in U.S. Pat. No. 5,130,949, issued Jul. 14, 1992, assigned to Atlantic Richfield Company and incorporated herein by reference, once the relationship of interval transit time to pore pressure is empirically determined, one may estimate pore pressures at locations away from an actual well by applying the relationship to interval transit times determined by common depth point (CDP) gathers of seismic survey data.
It is an object of the present invention to provide a method of determining pore pressures at locations away from the locations of actual wells, in order to perform well planning in advance.
It is a further object of the present invention to provide such a method which does not rely on regression estimation and curve fitting.
It is another object of the present invention to provide such a method that may be used on existing data in an automated manner.
Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.